21 1. Wave propagation Shear waves do not propagate in fluids. The ratio VP/VS is independent of the formation density so allows the calculation of Poisson’s ratio which is used to differentiate unconsolidated rocks (Poisson’s > 0.35) from consolidated rocks (Figure 1.2). Figure 1.2 Poisson’s ratio. In the three-dimensional reference system (x, y, z), the x-direction (horizontal) and the z-direction (vertical) represent the vertical plane of the acquisition profile, which contains the source and the receivers and is oriented perpendicular to the main axis of the structure. If the structure has no lateral variation in the y-direction, the structure is said to be cylindrical; variations in displacements along y are zero. On the assumption of a cylindrical structure, the propagation directions of all waves are contained within the plane of the acquisition profile. Particle displacements of the P-waves are in the (x, z) plane. S-wave particle motions are either in the plane of the profile – called SV-waves (vibrations in the vertical plane) – or perpendicular to the plane of the profile, called SH-waves (vibrations in the horizontal plane). The vibration and propagation directions of the various wave types, P, SV and SH-waves, are illustrated in Figure 1.1-B.
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